Revenue - Schedule of Revenue Disaggregates by Major Source (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Revenue from Contract with Customers [Line Items]
Revenues	$ 5,568,419
Sheet [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,666,220
Bar [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,090,147
Structural [Member]
Revenue from Contract with Customers [Line Items]
Revenues	396,697
Plate [Member]
Revenue from Contract with Customers [Line Items]
Revenues	427,630
Tubular Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	311,228
Rebar Fabrication [Member]
Revenue from Contract with Customers [Line Items]
Revenues	329,219
Other Steel Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	828,264
Raw Materials [Member]
Revenue from Contract with Customers [Line Items]
Revenues	519,014
Steel Mills [Member]
Revenue from Contract with Customers [Line Items]
Revenues	3,580,694
Steel Mills [Member] | Sheet [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,666,220
Steel Mills [Member] | Bar [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,090,147
Steel Mills [Member] | Structural [Member]
Revenue from Contract with Customers [Line Items]
Revenues	396,697
Steel Mills [Member] | Plate [Member]
Revenue from Contract with Customers [Line Items]
Revenues	427,630
Steel Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	1,468,711
Steel Products [Member] | Tubular Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	311,228
Steel Products [Member] | Rebar Fabrication [Member]
Revenue from Contract with Customers [Line Items]
Revenues	329,219
Steel Products [Member] | Other Steel Products [Member]
Revenue from Contract with Customers [Line Items]
Revenues	828,264
Raw Materials [Member]
Revenue from Contract with Customers [Line Items]
Revenues	519,014
Raw Materials [Member] | Raw Materials [Member]
Revenue from Contract with Customers [Line Items]
Revenues	$ 519,014